Method for producing a filter element

Abstract: A filter element includes a bundle (1) of hollow fibres made of a first polyolefin. The bundle (1) has at least one end sealed with a second polyolefin (2) having an average molecular weight in the range of 1,000 to 16,000. The filter element is produced by a method including the steps of fusing the second polyolefin (2) having a fusion point lower than that of the first polyolefin (1), attaching the fused second polyolefin (2) to at least one end of the bundle, allowing the attached second polyolefin to set, thereby providing the bundle with a sealed portion, and cutting off the leading end of the sealed portion.

Description: The present invention relates to a method for the production of a filter element, which is highly resistant to solvents and comprises a bundle of hollow fibres having at least one end sealed with a thermoplastic resin material. In conventional filter elements of this kind, at least one end of a bundle of hollow fibres is sealed with urethane adhesive. Use of such urethane adhesive leads to good workability when providing the at least one end of the bundle with a sealed portion. A filter module having such a conventional filter element contained in a housing exhibits sufficient durability to water. However, it is inferior in durability to an aqueous alcohol solution or organic solvents such as alcohol. In view of the above, filter elements each comprising a bundle of hollow fibres which has at least one end sealed with a thermoplastic resin material resistant to solvents have recently been developed. Such filter elements are disclosed, for example, in Japanese Patent Non-examined Publications No. 1-218605, No. 3-245826 and No. 4-63117. However, the use of a thermoplastic resin has disadvantages in (1) that the step of forming a sealed portion requires a treating temperature higher than in the case of using urethane adhesive in order to obtain fluidity of the resin, resulting in thermal deformation of the hollow fibres and possible destruction of the inside diameters of the hollow fibres depending on a combination with the material of which the hollow fibres are made, (2) that since thermoplastic resin has viscosity higher than that of urethane adhesive, it fails to propagate over the entire spaces between the adjacent hollow fibres even if it should gain fluidity, resulting in an incomplete sealed portion, and (3) that thermoplastic resin encounters volume shrinkage larger than that of urethane adhesive during its setting, resulting in an incomplete sealed portion having crazing and/or cavities. The first cited Japanese Publication discloses a method for the formation of a sealed portion of a bundle of hollow fibres by filling the inside of the ends of the hollow fibres with calcium carbonate so as not to induce thermal deformation, then sealing the spaces between the adjacent ends of the hollow fibres with fine particles of thermoplastic resin the same as the thermoplastic resin of which the hollow fibres are made, and fusion bonding the ends of the hollow fibres. This method requires the steps of removing the calcium carbonate by dissolving it in concentrated hydrochloric acid which will constitute waste matter and disposing of the waste matter, and encounters some difficulty in fusion bonding the ends of the hollow fibres liquid-tightly over the entire area using finely particulate sealing material and failure to fusion bond the centre portion of the bundle of hollow fibres. Thus, this prior art cannot eliminate the disadvantages (2) and (3) mentioned above. The second cited Japanese Publication discloses a method for the formation of a sealed portion of a bundle of hollow fibres, which adopts the step of causing the sealed portion to set from the lower part thereof to eliminate the aforesaid disadvantage (3) and uses, as a material for hollow fibres, engineering plastic generally exhibiting high resistance to heat, including polysulfone, polyether sulfone and polyamide and, as a sealing material, thermoplastic resin including polyolefin, polyvinyl chloride, nylon and polyester. This method requires that the fusion point of the material for hollow fibres should be much higher than that of the sealing material and cannot eliminate the aforesaid disadvantage (2). The third cited Japanese Publication discloses a method for the formation of a sealed portion of a bundle of hollow fibres, which uses a first polyolefin as a material for hollow fibres and, as a sealing material, a second polyolefin having a fusion point lower than that of the first polyolefin, and adopts the use of a suspension of the second polyolefin for sealing the ends of the hollow fibres. The products obtained in accordance with this method, however, are not stable in liquid tightness. Thus, this method produces no satisfactory results. US-A-3697635 describes a process for moulding onto the ends of a fibre bundle, in which the ends of the fibres are first strengthened by reinforcing mantles in a first moulding step, and are then sealingly incorporated into a moulded structure in a second moulding step. In the first moulding step the reinforcing mantles are not connected, but merely reinforce the fibre ends and space them from one another, thus creating a porous structure. Only in the second moulding step are the fibres provided with a sealed end portion. EP-A-0168558 teaches the encasement of the end of a bundle of thermoplastic tubes by injection moulding. The present invention has been proposed for the purpose of solving the problems mentioned above. An object of this invention is to provide a method for the production of a filter element highly resistant to solvents and chemicals. Another object of this invention is to provide a method for the production of the filter element at low cost. According to the invention, there is provided a method for the production of a filter element including a bundle of fibres made of a first polyolefin having a melting point, said method comprising the steps of (a) melting in a mould a second polyolefin having an average molecular weight in the range of 1000 to 16000 and a melting point lower than the melting point of said first polyolefin; (b) immersing at least one end of said bundle into the molten second polyolefin in said mould to attach the molten second polyolefin to said at least one end of said bundle; (c) removing said bundle from said mould; (d) allowing the attached second polyolefin to set, thereby providing said bundle with a sealed portion; (e) melting in a further mould a further quantity of said second polyolefin; (f) immersing said sealed portion of said bundle into said separately prepared molten second polyolefin contained in said further mould; (g) allowing said sealed portion to set; and (h) cutting off a leading end of said sealed portion. Embodiments of this invention will be described hereinbelow with reference to the accompanying drawings, in which: Figure 1 is an explanatory cross section showing one embodiment of a method for producing a filter element according to this invention; Figure 2 is a cross section showing the filter element produced by the method; and Figure 3 is an explanatory cross section showing another embodiment of a method for producing a filter element according to this invention. The present invention will now be described with reference to the illustrated embodiments. A bundle 1 of hollow fibres made of a first polyolefin is prepared. A second polyolefin having an average molecular weight in the range of 1,000 to 16,000 and a fusion point lower than that of the first polyolefin is fused in a mould 3 by heating the mould with a heating means 4. The difference between the fusion points of the first and second polyolefins may fall within 30°C. The fused second polyolefin 2 in the mould 3 is left standing until it has a temperature not higher than the fusion point of the first polyolefin while maintaining its fluidity. The prepared bundle 1 is then immersed in the fused second polyolefin 2 as shown in Figure 1. The bundle 1 is removed from the mould 3 immediately before the fused second polyolefin 2 loses its fluidity, and the second polyolefin attached to the bundle 1 is allowed to set. As a result, a filter element 9 having a sealed portion 10 of the second polyolefin is obtained as shown in Figure 2. The second polyolefin includes low-density polyethylene having an average molecular weight of 6,000 to 1000, high-density polyethylene having an average molecular weight of 7,000 to 1000, and polypropylene having an average molecular weight of 16,000 to 1000. Polypropylene or high-density polyethylene is preferable when a filter element being obtained is required to be sterilised with steam. Since the second polyolefin in a fused state exhibits lower viscosity than ordinary polyolefin, it can be fused by heating the mould 3 to a temperature of 200°C and holding the mould 3 at that temperature for several minutes, and the fused second polyolefin contains no air bubbles therein and is transparent and homogenous. Therefore, it is unnecessary to subject the fused second polyolefin to degassing under a reduced pressure. Where any problem may possibly arise when air is contained in the fused second polyolefin, the degassing step may be adopted. When the hollow fibres of the bundle 1 are made of high-density polyethylene, high-density polyethylene having a low molecular weight or low-density polyethylene having a low molecular weight is preferably used as the second polyolefin. When the hollow fibres are made of polypropylene, polypropylene having a low molecular weight, high-density polyethylene having a low molecular weight or low-density polyethylene having a low molecular weight is preferably used as the second polyolefin. These combinations of the first and second polyolefins can avoid thermal deformation of the bundle 1 of hollow fibres and produce a completely sealed portion 10 having the second polyolefin propagating entirely over at least one end of the bundle 1. As described above, the bundle 1 is removed from the mould 3 immediately before the fused second polyolefin 2 loses its fluidity, and the second polyolefin attached to the bundle 1 is allowed to set outside the mould 3. The removal of the bundle 1 from the mould 3 is carried out preferably when the temperature of the mould 3 is in the range between the fusion point of the second poloyolefin and a temperature not exceeding the temperature 10°C higher than the fusion point of the second polyolefin. By so doing, the second polyolefin can firmly be attached to the bundle 1. In addition, since the second polyolefin attached to the bundle 1 is allowed to set outside the mould 3, there is no possibility of the sealed portion 10 suffering from crazing and/or cavities by virtue of volume shrinkage of the second polyolefin. Therefore, a partially finished filter element having at least one end sealed with the second polyolefin can be obtained. In order to obtain a filter element having a more completely sealed portion, the filter element 9 shown in Figure 2 is inserted at its sealed portion 10 into another mould 7 having a bottomless nylon cup 6 attached to the inside thereof and a heating means 8 attached to the outside thereof as shown in Figure 3. The mould 7 contains either fused polyolefin which is the same as the second polyolefin or a fused mixture of the second polyolefin and ordinary polyolefin having an average molecular weight larger than that of the low molecular weight polyolefin. The second polyolefin may have a low molecular weight. The filter element 9 is removed from the mould 7 and allowed to set in the same manner as described above. The filter element 9 thus obtained is associated with a given support cylinder via an O-ring or the like into a filter module. A given support cylinder made of ordinary polyolefin of the same type as the low molecular weight polyolefin may be inserted into the mould 7. In this case, the filter element being obtained can be used as a filter module. In a case where the mould 7 contains a fused low molecular weight polyolefin and where a given support cylinder is not inserted into the mould 7, there is a possibility that a filter element being obtained has a sealed portion liable to break when receiving a large external impact. This can be avoided by using the aforementioned fused mixture instead of the fused low molecular weight polyolefin. Examples of the ordinary polyolefin include low-density polyethylene, linear low-density polyethylene, high-density polyethylene and polypropylene. The content of the ordinary polyolefin in the mixture is preferably in the range of 10 to 30% by weight. According to the method of this invention, even when hollow fibres constituting a bundle are made of engineering plastic having a higher fusion point than polypropylene, use of the low molecular weight polyolefin can eliminate otherwise possible thermal deformation of the bundle and provide a completely sealed portion having the low molecular weight polyolefin fully propagating the spaces between the adjacent hollow fibres. The advantages of the present invention will become more apparent from a working example and comparative experiments which will be described hereinafter. Example 1 A bundle 1 was prepared by bundling 3,000 hollow fibres made of polypropylene. A mould 3 containing high-density polyethylene having an average molecular weight of 3,000 was heated by a heater 4 until the fused polyolefin had no air bubbles therein. The application of heat was then stopped and the mould 3 was left standing. As soon as the temperature of the mould 3 was 160°C, the bundle 1 was inserted into the fused polyolefin and left standing. When the mould temperature became 120°C, then the bundle 1 was removed from the mould 3 to obtain a partially finished filter element 9 having a sealed portion 10 at one end thereof. Another mould 7 having a bottomless nylon cup 6 attached fast to the inside thereof and a mixture 5 of high-density polyethylene having an average molecular weight of 3,000 and 20% by weight of straight-chain polyethylene (MFR80 as specified under ASTM) contained inside the cup 6 was heated by a heater 8 until the fused mixture 5 had no air bubbles therein. The application of heat was then stopped and the mould 7 was left standing. As soon as the mould temperature was 130°C, the sealed portion 10 of the filter element 9 was inserted into the fused mixture 5. When the temperature of the mould 7 became 100°C, then the filter element 9 having the cup 6 attached to the further sealed portion thereof was removed from the mould 7 and allowed to set. After the further sealed portion had set sufficiently, the attached cup 6 was removed. As a result, a filter element 9 was obtained having the end portion 10 sealed completely with polyolefin. Neither crazing nor cavities could be observed in the sealed portion 10. The leading end of the sealed portion 10 of the filter element 9 thus obtained was cut off to form an open surface of the filter element 9. The filter element 9 was fixed to the inside of a pressure container with the sealed portion 10 sealed with an O-ring. Then, water pressure of 4 kg/cm 2 was applied to the filter element 9. However, no leak of water from between the hollow fibres could be found at the cut open surface of the filter element 9. With the filter element 9 fixed inside the pressure container, 1O l of ethyl alcohol was filtered. Then, the filter element 9 was removed from the pressure container, dried in an oven heated to 60°C, and fixed again to the inside of the pressure container. Thereafter water pressure of 4 kg/cm 2 was applied to the filter element 9. However, no leak of water could be found at the cut open surface of the filter element 9. Although this test was repeated five times, no leak of water could be recognised at the cut open surface of the filter element 9. Comparative Experiment 1 A bundle 1 was prepared in the same manner as in Example 1 and the same procedure as in Example 1 was taken until the insertion of the bundle 1 into the fused polyolefin. When the temperature of the mould 3 became room temperature, the bundle 1 is removed from the mould 3 to obtain a filter element 9 having a sealed portion 10 at one end thereof. The leading end of the sealed portion 10 of the filter element 9 thus obtained was cut off to form an open surface of the filter element 9. As a result, crazing was recognised in the cut open surface. The filter element 9 was fixed in the same manner as in Example 1 and subjected to water pressure of 4 kg/cm 2 . As a result, leak of water from between the hollow fibres was found at the cut open surface of the filter element 9. Comparative Experiment 2 A bundle 1 was prepared in the same manner as in Example 1 and a filter element 9 having a sealed portion 10 was obtained in the same manner as in Example 1 except that urethane adhesive was used as the sealing material. The leading end of the sealed portion 10 of the filter element 9 thus obtained was cut off to form an open surface of the filter element 9. As a result, crazing was recognised in the cut open surface. The filter element 9 was fixed in the same manner as in Example 1 and subjected to water pressure of 4 kg/cm 2 . As a result, no leak of water from between the hollow fibres could be found at the cut open surface of the filter element 9. The same test as in Example 1 effecting filtration of ethyl alcohol and application of water pressure of 4 kg/cm 2 was repeated. As a result, after the second and subsequent tests, leak of water from between the hollow fibres was recognised at the cut open surface of the filter element 9. As is clear from Example 1 and Comparative Experiments 1 and 2, according to the present invention, it is possible to provide a filter element made of polypropylene applicable to filtration of not only water but also an organic solvent, more inexpensive than filter elements made of an engineering plastic material, and excellent in quality and reproducibility without forming crazing and/or cavities in its sealed portion.

Claims: A method for the production of a filter element (9) including a bundle (1) of fibres made of a first polyolefin having a melting point, said method comprising the steps of (a) melting in a mould (3) a second polyolefin having an average molecular weight in the range of 1000 to 16000 and a melting point lower than the melting point of said first polyolefin; (b) immersing at least one end of said bundle (1) into the molten second polyolefin (2) in said mould to attach the molten second polyolefin to said at least one end of said bundle; (c) removing said bundle from said mould; (d) allowing the attached second polyolefin to set, thereby providing said bundle with a sealed portion (10); (e) melting in a further mould (7) a further quantity of said second polyolefin; (f) immersing said sealed portion (10) of said bundle into said separately prepared molten second polyolefin (5) contained in said further mould (7); (g) allowing said sealed portion to set; and (h) cutting off a leading end of said sealed portion. A method according to claim 1, characterised in that said further mould (7) further contains molten ordinary polyolefin admixed with said separately prepared molten second polyolefin (5). A method according to claim 2, characterised in that said ordinary polyolefin is low-density polyethylene, linear low-density polyethylene, high-density polyethylene or polypropylene. A method according to any preceding claim, characterised in that said first polyolefin is polypropylene or high-density polyethylene. A method according to any preceding claim, characterised in that the difference between the melting point of said first polyolefin and that of said second polyolefin falls within 30°C.

Inventors: MONZEN TAKASHI, WATANABE MASAHARU, MONZEN, TAKASHI, WATANABE, MASAHARU, MONZEN, TAKASHI, C/O KITZ CORPORATION, WATANABE, MASAHARU, C/O KITZ CORPORATION
Applicants: KITZ CORP, SHINSOZAI SOGO KENKYUSHO KK, KITZ CORPORATION, MATERIAL ENGINEERING TECHNOLOGY LABORATORY, INC.
Application year: 1994
Main classification: B01D63
IPC codes: B29C41, B01D63, B01D71, B29C70